The present invention is directed to a process for dehydrocoupling toluene using a modified zeolite composition.
Styrene is currently commercially produced from benzene in a two-step process. In the first step benzene is alkylated with ethylene to form ethylbenzene, and in the second step, the ethylbenzene is dehydrogenated to form styrene.
One of the known alternative routes for forming styrene involves the oxidative coupling of toluene to form 1,2-diphenyl ethylene (stilbene) followed by the disproportionation of the stilbene with ethylene in the presence of a catalyst to form styrene. The economic significance of the overall process scheme of the toluene-stilbene route resides in the fact that styrene can be produced from 0.5 mole of ethylene and one mole of toluene. This compares with the conventional ethylbenzene route wherein styrene is produced from one mole of ethylene and one mole of benzene. In light of the rising costs of benzene and ethylene and the environmental problems of benzene, toluene-based processes will become a more attractive route than the existing benzene-based process for styrene manufacture.
Another alternative route to styrene from toluene involves the alkylation of toluene with methanol by contact of these reactants with X- or Y-type zeolites, as described in Yashima et al in the Journal of Catalysis, Vol. 26, 303-312 (1972). However, since zeolites are capable of catalyzing a variety of reactions and therefore produce a variety of by-products, the selectivity of the toluene to styrene is very low when conducting the process in accordance with Yashima et al.
In an effort to improve the selectivity of the toluene/methanol alkylation reaction to styrene, Unland et al, U.S. Pat. No. 4,140,726 describe the use of an X- or Y-type zeolite which has been modified by a cation exchange with one or more of potassium, rubidium and cesium and impregnation with boron or phosphorus. While the modification of the zeolite improves the selectivity to styrene to some extent, the only useable by-product of the reaction is ethylbenzene, while a substantial amount of unuseable by-products are also formed. Thus, from the data reported in Unland et al, the maximum total eventual selectivity to styrene which could be achieved even if one dehydrogenates all the ethylbenzene by-product formed is only about 54% (see Table 5, Run 2 of this patent).
In contrast, the process of the present invention does not employ a side alkylation reaction of toluene with methanol. Instead, the process of the present invention relies on the dehydrocoupling of toluene to form inter-alia stilbene, and diphenylethane.
In addition to its utility as an intermediate in production of styrene, stilbene, because of its unsaturated character, is very reactive and may be employed in various organic syntheses. Derivatives of stilbene are useful in the production of chemicals which may be used in the manufacture of dyes, paints, and resins. It is also useful in optical brighteners, in pharmaceuticals and as an organic intermediate.
Thus, there is substantial economic incentive to develop an economical process for producing stilbene.
The ideal reaction to stilbene from toluene is the direct dehydrocoupling reaction summarized as follows: ##STR1## Such a selective reaction in practice is difficult to achieve. More often, the overall reaction involves the dehydrocoupling of toluene to stilbene as well as bibenzyl. Bibenzyl however can be dehydrogenated to stilbene. Furthermore when the catalysts of the present invention are employed, benzaldehyde is formed as a by-product. However, benzaldehyde can be subsequently converted to stilbene in high yield by conventional methods such as by the McMurray reductive coupling reaction using hydrogen. Thus, a commercial process for producing stilbene could include an overall reaction scheme summarized as follows: ##STR2##
The stilbene can then be converted to styrene by the well known metathesis reaction with ethylene (see for example U.S. Pat. Nos. 3,965,206 and 4,117,021), or the stilbene can be used directly for purposes described herein above.
The reaction of Equation 1, employing oxygen as the oxidant in the absence of a catalyst, is extremely inefficient because of the preponderance of non-selective fre-radical reactions leading to complete combustion of the hydrocarbons and the formation of oxygenated by-products. Consequently, attempts have been made to improve the selectivity of the reaction using oxidants, such as metal or non-metal oxides which function in a stoichiometric mode as stoichiometric reactants providing lattice oxygen which is depleted during the reaction. Because of the oxygen depletion of metal oxide stoichiometric oxidants, their use requires that they be either very inexpensive and therefore disposable, or they must be capable of being regenerated by replacing the lattice oxygen lost during the reaction. Since many of the conventional stoichiometric metal oxide oxidants are expensive, their use requires extensive plant equipment and engineering design to provide proper regeneration. This has led to two alternative approaches; namely, fixed bed and fluidized bed systems. In the fixed bed system, two or three reactors with staggered cycles typically are employed to achieve continuous operation. This system is very costly in terms of plant equipment. In the fluidized system a single reactor can be employed and a portion of the metal-oxide can be constantly removed, regenerated, and returned to the reactor. Fluidized systems, however, lead to attrition of the metal oxide and in many instances the metal of the metal oxide can be lost as fines which foul the interior of the reactor.
To reduce the need for frequent regenerations, prior art metal oxygen compositions can also be operated in the catalytic mode.
In the catalytic mode of operation, oxygen or an oxygen-containing gas such as air or oxygen-enriched air is reacted with toluene in an amount sufficient for the dehydrocoupling reaction, the reaction being catalyzed by, and conducted in the presence of the metal oxygen composition.
Metal oxygen, compositions which operate in the catalytic mode, however, exhibit reduced selectivity and/or conversion relative to their use in the stoichiometric mode. For example, Example 6 in U.S. Pat. No. 4,091,044 illustrates the use of a Sb/Pb/Bi oxide oxidant in a stoichiometric mode. When contacted for 1 minute at 580.degree. C. with a steam toluene feed (Run 3, Table 6) the conversion is 47.3% and a selectivity for cis and trans stilbene plus bibenzyl is 81.2%. However, after 7 minutes reaction time (Run 6, Table 6) the conversion drops to 9.7% at a corresponding selectivity of 87.5%. The substantial drop in conversion over a period of 5 minutes indicates that the oxidant is quickly deactivated. It is for this reason that the oxidant is typically regenerated for 30 to 60 minutes after each one-minute run (see Example 1, lines 34 et. seq.).
However, in Example 7 the metal oxygen composition is employed in the catalytic mode by adding air to the feed stream. While deactivation is minimal in terms of conversion after 70 minutes on-stream time, the conversion of toluene is 30%, selectivity to stilbene plus DPE is 57.6% and the selectivity to benzene plus CO.sub.2 is increased from 7.1% (in Table 6, Run 6, stoichiometric mode) to 36.7% (in Table 7, Run 3). It is a disadvantage of this process that the by-products of benzene and CO.sub.2 are formed in high amounts and are unuseable for subsequent conversion to stilbene.
Thus, it would be a significant improvement over conventional prior art metal oxygen compositions if a toluene dehydrocoupling catalyst could be identified which did not have to be constantly regenerated, i.e., could be used in the catalytic mode, and which also produced high amounts of stilbene or by-products which could be eventually converted to stilbene.
Representative examples of conventional metal oxide oxidants and/or catalysts are disclosed in U.S. Pat. Nos. 3,694,518; 3,739,038; 3,868,427; 3,965,206; 3,980,580; 4,091,044; 4,183,828; 4,243,825; 4,247,727; 4,254,293; 4,255,602; 4,255,603; 4,255,604; 4,268,703; 4,268,704; 4,278,824; 4,278,825; and 4,278,826 all assigned to Monsanto. These patents disclose numerous metal/oxide compositions which can be prepared by a variety of methods. None of these methods include the use of the modified zeolite catalysts of the present invention. While alumina-silica is disclosed as possible support in many of these patents, alumina-silica is amorphous in nature and is not conventionally understood to include zeolites which are crystalline in nature.
British Patent Specification No. 1,259,766 discloses a process for the oxidative coupling, of compounds which include toluene, in the presence of oxygen and a catalyst composition consisting of (1) at least one oxide of an element selected from the metals of Groups 2a (i.e., Be, Mg, Ca, Sr, Ba, Ra), 3a (i.e., B, Al, Ga, In, Tl), 4a (i.e., C, Si, Gc, Sn, Pb), 1b (i.e., Cu, Ag, Au), 2b (i.e., Zn, Cd, Hg), 3b (i.e, Sc, Y, La, Ac), 4b (i.e., Ti, Zr, Hf), 7b (i.e., Mn, Tc, Re), and 8 (i.e., Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt) of the Periodic Table, Bi, Cr, W, Te, Se, P, As, and Sb, and (2) at least one oxide, hydroxide, or salt of a metal of Group 1a, i.e., Li, K, Rb, Cs, and Fr (but excluding sodium oxide). The use of zeolites in accordance with the process of the present invention is not disclosed in this patent.
Commonly assigned U.S. patent application Ser. No. 387,693 filed June 11, 1982, by H. Teng, I, Huang, and H. Labowsky describes an organic method for preparing metal oxygen compositions for the dehydrocoupling of toluene. While this application discloses that such metal oxides can be applied to zeolites in general, the specific combination of exchange and promoter materials as well as the particular type of zeolites employed in accordance with the process of the present invention is not disclosed.
U.S. Pat. No. 4,192,961 discloses a process for the dehydrodimerization of toluene with oxygen to form diphenylethane and stilbene in the presence of a bismuth oxide catalyst at temperatures of 400.degree. to 600.degree. C., and the subsequent conversion of stilbene with ethylene to styrene in the presence of a catalyst consisting of chromium oxide, tungsten oxide, an alkali metal oxide on a silica or aluminosilicate support. The conversions of toluene are extremely low, e.g. about 4%. Furthermore, the use of zeolites for either step in the process is not disclosed in this patent.
Other literature relating to the dehydrocoupling of toluene include W. Ger. Offleg. No. 2,500,023 which discloses the use of 20% PbO on Mg-Al.sub.2 O.sub.3 in a stoichiometric mode and a selectivity to stilbene and diphenylethane of 67%, and 1.8% respectively at a conversion of 41%.
U.S. Pat. No. 4,117,021 discloses the use of ZnO and PbO as catalysts for dehydrocoupling toluene.
The search has therefore continued for processes capable of dehydrocoupling toluene at relatively high toluene conversions and overall selectivity to stilbene which do not have to be frequently regenerated to maintain their activity. The present invention is a result of this search.